Hydrogen in porous Vycor glass

Hydrogen in porous Vycor glass (pore radius ${\mathit{R}}_{\mathit{p}}$=3.0 nm) has been investigated with a torsional oscillator technique at 7.5 K ≤T≤ 22 K. ${\mathrm{H}}_2$ molecules which are adsorbed in Vycor at T≳${\mathit{T}}_3$ (${\mathit{T}}_3$, triple point of bulk ${\mathrm{H}}_2$) leave the Vycor when decreasing the temperature to below a characteristic value ${\mathit{T}}_{\mathit{c}}$≤${\mathit{T}}_3$; ${\mathit{T}}_{\mathit{c}}$ depends on the amount of ${\mathrm{H}}_2$ in the Vycor. This interpretation of the data is supported by simultaneous measurements of the ${\mathrm{H}}_2$ vapor pressure. A similar phenomenon is observed with a capacitor filled with Vycor into which ${\mathrm{H}}_2$ is condensed. We conclude that the free energy of solid ${\mathrm{H}}_2$ in the Vycor is larger than that of bulk solid ${\mathrm{H}}_2$. Information on the free energy of ${\mathrm{H}}_2$ confined in the Vycor is important to understand the depression of its freezing temperature in restricted geometries. We also discuss the properties of solids and the depression of their freezing temperature in restricted geometries. © 1996 The American Physical Society.